Applications solder replacement adhesives

Adhesives used for screen or stencil printing in surface-mount applications are generally electrically insulative types whose functions are mechanical attachment and thermal dissipation. However, electrically conductive, silver-filled epoxies have been used for many years as ohmic contact adhesives to interconnect bare-chip devices in hybrid microcircuits and are used as solder replacements for surface mounting of components on printed-circuit boards. Regardless of their... 

Only very limited work has been conducted to investigate the high-frequency behaviors of IGA joints. Felba et al. [82] investigated a formulation of isotropically conductive adhesives that performed well as a solder replacement in microwave applications. The study involved various different adhesive base materials and several types of main (silver flakes, nickel, and graphite) and additional (soot and silver semiflake powder) filler materials. In order to assess the usefulness of a given adhesive formulation, an additional gap in the gold strip of a standard microstrip bandpass... 

Over the past decades, development and use of electrically conductive adhesives have expanded in the microelectronics industry. The major applications are die attachment, liquid crystal display and surface-mounted assembly of packaged components on printed wiring board. Recently, isotropic conductive adhesives (ICA s) have been widely used in electronics applications to replace lead-based soldering due to toxicity of the lead in Sn-Pb solders [1]. 

Conductive adhesives are one of the feasible alternatives to lead for electronics assembly. Isotropically conductive adhesives are suitable for standard pitch (50- to 100-mil) surface-mounted components and numerous commercial materials are available (see commercial suppher Ksting, Section VI.E). Anisotropically conductive adhesives are more suited to flex to rigid connections, fine pitch components (15- to 20-mil pitch), and flip-chip assembly (4- to 12-mil pitch) [22]. Adhesives are not ready to replace solder throughout the electronics industry, however, due to questions that remain concerning the reliability of electrical interconnections. Their implementation is currently limited to low-cost applications using polyester substrates and specialty appHcations where solder cannot be used. Additionally, the lack of equipment for large-volume assembly with anisotropically conductive adhesives, which require the simultaneous appUcation of heat and pressure, impedes the acceptance of these promising materials. 

Electrically conductive adhesives, primarily silver-filled epoxies, are finding uses as replacements for solder in surface-mounting components on printed-circuit boards and in flip-chip attachments. There are several driving forces for this application, a major one being the trend to eliminate lead and tin-lead solders because they may be health hazards. Also associated with the use of solder, is the need to eliminate ozone-depleting solvents presently used to clean and remove flux residues. Electrically conductive polymer... 

Another application is in ferroelectric devices used to bond electrode terminals to the crystals in stacks. These adhesives replace solders and welds where crystals tend to be deposited by soldering and welding temperatures. Bonding of battery terminals is another application when soldering... 

Electrically conductive adhesives are used for such applications as bonding lead wire to electrodes, solder paste replacement in the assembly of SMD, die attachment, and repair of conducting paths. Adhesives such as epoxies or acrylics are highly filled (typically 80%) with conductive metal particles. Metals such as nickel and copper cannot be used because of their susceptibility to oxidation. Precious... 

The adhesive has minimal function after the soldering application, but it must not reduce the reliability of the circuit by becoming conductive during environmental exposure, or put stress on the components during thermal cycling or mechanical handling. On the other hand, if component replacement is necessary, the adhesive should soften sufficiently at soldering iron temperature to allow part removal. 

Another application is in ferroelectric devices used to bond electrode terminals to the crystals in stacks. These adhesives replace solders and welds where crystals tend to be deposited by soldering and welding temperatures. Bonding of battery terminals is another application when soldering temperatures may be harmful. Conductive adhesives form joints with sufficient strength, so they can be used as structural adhesives where electrical continuity, in addition to bond strength, is required, as in shielded assemblies [37]. Sharpe [38] has pubhshed an excellent comprehensive review of electrically conductive adhesives. 

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